The manufacturing of precision parts requires measuring methods for recording the geometry and the state of the parts in order to ensure the quality of the corresponding parts. Optical measuring methods such as image acquisition and image evaluation, interferometry, particularly white-light interferometry, makes an important contribution here.
The principle of the white-light interferometer is based on the fact that a short-coherent light source is used for the illumination of an imaging system. In addition to the normal imaging optics, the imaging system has a reference arm which is traversed by a portion of the irradiated light. If the transit path of the light Λo in the object arm and the transit path in the reference arm ΛR now have a path difference that is less than the coherence length Ic of the light, i.e.|ΛR−ΛO|<IC  (1)then the light fields brought together again can exhibit a measurable interference. This is utilized in that, during the measurement, the path difference of the light fields, defined by the shift of the object or the reference element along the optical axis, is altered. At the same time, the intensity of the reunited light fields is measured on a detector, usually a CCD camera, measuring in planar fashion. Since a constructive or destructive interference can only take place within the coherence length of the white-light source, the pixel-by-pixel evaluation of the intensity modulation produced by the interference, the intensity correlogram, supplies clear information concerning height for each individual pixel. Carried out for the entire pixel field, this results in complete height information for the object.
Commercial white-light interferometers typically have the following specifications:
The height resolution Δz is given by the average utilized wavelength of the light λm, the coherence length Ic and the type of correlogram evaluation algorithm. Typical parameters such as λm=600 nm, Ic=2 μm permit values of Δz=1 nm.
The lateral resolution δ is equal to that of a conventional imaging system and, in principle, is limited by λm and the numerical aperture NA of the imaging optics.δ≧0.61 λm/NA  (2)
The maximum measurable total height difference zmax is determined by the technical feasibility of producing a path difference in the reference arm and object arm that is guided precisely over the entire distance. Regulated piezosystems today permit values of zmax≧400 μm.
Conventional interferometers, particularly white-light interferometer systems, can be used for the tasks described above when the location to be measured is easily accessible and has a predominantly flat geometry. If this is not the case, interferometers are used which have special optics adapted to the object to be measured. However, these interferometers have the disadvantage that undercuts on the object to be measured lie in the shadow area of the illumination and therefore cannot be recorded. To measure these surfaces, the object must be dismounted and measured in a second measuring operation.